Efficient railroad transportation systems require that railroad rails withstand the demands of high-axle loads, acceleration and deceleration friction and stress, and high usage. Rail comprises a head, a base, and a web between the head and base. The head comprises an upper gage corner on each side of the top of the head. Rolling fatigue and damage occurs on the top surface of the head of straight rail and typically one of the two upper gage corners of curved rail and is a constant maintenance issue requiring periodic rail replacement.
Fracture toughness, or toughness, is a term used in the art to describe steel's resistance to cracking. Steel having a high toughness while maintaining ductility is less prone to fatigue cracking. The steel will be more fracture resistant to impact loads but more prone to wear and abrasion under sliding loads. Hardness is a term used in the art to describe steel's resistance to deformation. A steel having a high hardness while retaining ductility is less prone to wear and abrasion. Ideal steel for rail would be one that has a high toughness and a high hardness.
In its simplest form, steel is composed of a mixture of iron (Fe) and carbon (C). During the production process, the mixture is cooled from about 1000.degree.C. to 723.degree.C. For a mixture of iron and carbon with a 0.83 wt % of C, at 723.degree.C., the iron and carbon transforms into a solid solution of alternating lamellae of soft iron, known as ferrite, and very hard iron carbide, known as cementite. The resulting steel has an all pearlite structure and is referred to as eutectoid. Eutectoid steel is characterized as having the highest tensile strength as compared with other pure iron-carbon ratios.
A pure iron and carbon mixture having less than 0.83 wt % of C results in pearlitic steel that is hypo-eutectoid. That is, when the iron and carbon mixture is cooled from about 1000.degree.C. to 723.degree.C., some of the mixture transforms into ferrite. At 723.degree.C., the remaining iron and carbon transforms into a solid solution of pearlite. If the steel is cooled very slowly, the first to transform ferrite will diffuse into the ferrite layers of the pearlite. Common steel producing techniques compromise the cooling time for efficiencies and through-put of the mill, resulting in a cooling process that is too fast for complete diffusion. Hypo-eutectoid pearlitic steel approaching 0.83 wt % of C is characterized as having good resistance to wear because of the hard cementite in the pearlite and some degree of toughness as a result of the ferrite's ability to flow in an elastic/plastic manner.
Pure iron and carbon mixtures having a decreasing amount of wt % of C below 0.83 wt % will produce a steel having an increasing amount of ferrite, as more ferrite will form before the mixture transforms into pearlite. This will produce steel of increasing toughness and decreasing hardness.
Pure iron and carbon mixtures having more than 0.83 wt % of C are referred to as hyper-eutectoid. That is, when the iron and carbon mixture is cooled from about 1000.degree.C. to 723.degree.C., some of the mixture transforms into cementite. At 723.degree.C., the remaining iron and carbon transforms into pearlite. Therefore, hyper-eutectoid steel comprises pearlite and cementite.
Pure iron and carbon mixtures having an increasing amount of wt % of C above 0.83 wt % will produce a steel having an increasing amount of cementite, as more cementite will form before the remaining iron and carbon transforms into pearlite. This will produce steel of increasing hardness and decreasing toughness. Hyper-eutectoid pearlitic steel is characterized as being very hard and therefore wear resistant, but brittle.
Railroad rail would benefit from being made from steel having both high toughness and high hardness. Increasing amounts of carbon along with alloying agents and manufacturing processing parameters are used in an attempt to retain the toughness of a hypo-eutectoid steel yet increase the hardness. Alloying can be used to produce a finer structure pearlite that will increase hardness as well as suppress the formation of cementite. The speed in which the steel is cooled from a high roll-forming temperature through the eutectoid temperature, 723.degree.C., and finally to ambient temperature has a dramatic effect on the formation of the pearlitic structure. One approach that has been used in the art is the development of steel alloys containing chromium, silicon and manganese. Though the resulting rails exhibit good performance in terms of wear and fracture resistance, the industry is striving for better performance. Further, the success of achieving an eutectic steel railroad rail with a carbon content higher than 0.90 wt % has been allusive.